Radiation crosslinked block copolymer blends with improved impact resistance

ABSTRACT

Polymer blends having high impact resistance after mechanical working are produced by blending together a non-elastomeric monovinylidene aromatic polymer such as polystyrene with an elastomeric copolymer, such as a block copolymer of styrene and butadiene, in the form of crosslinked, colloidal size particles.

BACKGROUND OF THE INVENTION

This invention relates to a method for making improved high impactpolymer blends, particularly high impact blends of monovinylidenearomatic polymers with elastomeric polymers.

Heretofore high impact polymer compositions, such as impact polystyrenecompositions, have been formed by first making an elastomeric polymersuch as polybutadiene or styrene/butadiene copolymer, adding theelastomeric polymer to monovinylidene aromatic monomer such as styrene,and then polymerizing the monovinylidene aromatic monomer to form agraft copolymer comprising a polymer of the monovinylidene aromaticmonomer grafted onto the polybutadiene. Also heretofore elastomericcopolymers have been mechanically blended with a monovinylidene aromaticpolymer to improve impact resistance of the latter.

In the latter instance involving mechanical blending of the polymers,experience has shown that one or more properties of the polystyrene suchas impact strength cannot be improved without substantial sacrifice ofone or more other, often equally important, properties such as tensilestrength and flexural modulus. In order to achieve improvement in impactstrength while eliminating or at least minimizing the loss of otherphysical properties, low amounts of high performance rubbers such as theblock copolymers of styrene and butadiene have been blended with thepolystyrene.

However, even in view of such improved blends, it remains highlydesirable to provide blends having further improved impact strengthwhile retaining tensile strength and modulus comparable to polystyrene.

SUMMARY OF THE INVENTION

In accordance with the present invention, a polymer blend comprising anon-elastomeric monovinylidene aromatic polymer and an elastomericcopolymer of conjugated diene and monovinylidene aromatic monomer, saidblend having improved impact resistance even after mechanicalprocessing, is provided by the following method. This novel methodcomprises the steps of subjecting the elastomeric copolymer in the formof colloidal size particles to conditions sufficient to crosslink thecopolymer, and combining the crosslinked copolymer particulate with themonovinylidene aromatic polymer. For the purposes of this invention, theterm "elastomeric" means a substance capable of being extended to twiceits own length at 68°C by applying a stress and on release of the stressreturns with force to approximately its original length.

Surprisingly, the impact resistance of the polymer blend prepared by theaforementioned method and then mechanically worked as is typical inpolymer fabrication is superior to the impact resistance of a polymerblend of the same components which is prepared under conditions which donot crosslink the elastomeric copolymer.

The resultant high impact polymer blends produced by the method of thisinvention are useful in generally any application known for high impactpolymers such as rubber-modified polystyrene compositions. For example,the polymer blends produced in accordance with this invention can beemployed in making appliance housings, furniture, luggage shells, toteboxes, architectural trim, translucent covers for light fixtures and thelike.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The non-elastomeric monovinylidene aromatic polymer is intended to meanone or more polymerized monovinylidene aromatic compounds of the benzeneseries such as styrene, α-methylstyrene, t-butylstyrene, vinyl toluene,vinyl xylene, ethyl vinyl benzene, chlorostyrene, ethyl vinyl toluene,isopropenyl toluene, diethyl vinyl benzene. Also included are copolymersof at least about 70 weight percent of one or more of suchmonovinylidene aromatic monomers with up to 30 weight percent of otherethylenically unsaturated monomers copolymerizable with themonovinylidene aromatic compound such as acrylonitrile,methacrylonitrile, acrylic acid, methacrylic acid, maleic anhydride,vinyl chloride, vinylidene chloride, methyl methacrylate, ethylacrylate, acrylamide, butadiene, isoprene and the like. The preferredmonovinylidene aromatic polymers are polystyrene, polyvinyl toluene,styrene/vinyl toluene copolymers, and copolymers of about 70 to about 75weight percent styrene and about 25 to about 30 weight percent ofacrylonitrile. Such preferred polymers have molecular weights andphysical property characteristics such that they can be employed alonefor fabrication by molding, extrusion, etc. into articles of hardness,toughness and utility as exemplified by polystyrene presently in suchcommercial uses. Accordingly, such preferred polymers have molecularweights of at least 50,000, by the Staudinger viscosity method(Schildknecht, "Vinyl and Related Polymers," New York, Wiley, 1952, pp.30-31). Such polymers may be prepared by any of the well-knownpolymerization processes disclosed in the prior art.

The elastomeric copolymers employed in the present invention arepreferably polyvinylaromatic-polydiene block copolymers having an A-Bconfiguration and polyvinylaromatic-polydiene-polyvinylaromatic blockcopolymers having an ABA configuration and hereinafter referred to asA-B block copolymers and A-B-A block copolymers. Other block copolymersof polyvinylaromatic and polydiene blocks are suitably employed as theelastomeric block copolymer in the present invention. It is furtherunderstood that blends of the foregoing block copolymers may also besuitably employed. Preferably the block copolymers have inherentviscosities of from about 0.6 to about 2.5 deciliters/gram (0.6 gram ofcopolymer per 100 grams of toluene at 25°C). Such block copolymerscomprise from about 80 to about 20 weight percent of A-block, preferablyfrom about 70 to about 30 weight percent, and from about 20 to about 80weight percent of B-block, preferably from about 30 to about 70 weightpercent.

The A-blocks are non-elastomeric polymer blocks of predominantlymonovinylidene aromatic monomer or monomers such as styrene,α-methylstyrene, ar-methylstyrene, α,ar-dimethylstyrene,ar-t-butylstyrene, ar-chloro- and ar,ar-dichlorostyrene, ar-bromo- andar,ar-dibromostyrene, vinyl naphthalene and others as describedhereinbefore. The A-block advantageously has a molecular weightgenerally in the range of from about 10,000 to about 150,000.

The B-blocks are elastomeric polymer blocks of predominantly aliphaticconjugated diene or dienes such as 1,3-butadiene, isoprene,methylisoprene, and the like. The B-block advantageously has a molecularweight generally in the range of from about 40,000 to about 300,000.

The block copolymers can be made by processes involving the sequentialformation of the individual blocks. More specifically, the copolymersare formed, for example, by (1) polymerizing monovinylidene carbocyclicaromatic monomer such as styrene in the presence of a lithium basedinitiator or a Ziegler type catalyst and (2) introducing aliphaticconjugated diene such as butadiene and permitting polymerization tocontinue until all of the monomer is consumed. Other suitable processesfor making the block copolymer are described in Journal of PolymerScience, Part C, Polymer Symposia, No. 26 1969, Interscience Publishers.

In addition to the foregoing block copolymers, elastomeric graftcopolymers of monovinylidene aromatic monomer and conjugated diene arealso suitably employed as the elastomeric copolymer.

In the practice of the invention, the elastomeric copolymer, singularlyor as a blend of the elastomeric copolymer and another elastomericpolymer such as a similar copolymer or polybutadiene, usually in theform of a mass or a polymer solution is preferably converted to acolloidal-size particulate in the following manner. The elastomericcopolymer is first dissolved in an organic liquid. Exemplary solventsare the cyclic alkanes such as cyclopentane, cyclohexane, cycloheptane,and the like; aromatic hydrocarbons such as benzene, toluene,naphthalene and xylene; acyclic alkanes such as n-pentane, n-hexane,n-heptane, n-octane, iso-octane; other solvents such as methyl ethylketone, tetrahydrofuran, and methylene chloride and mixtures thereof.

Solutions of the elastomeric copolymer advantageously contain from about5 to about 30 weight percent of the block copolymer, preferably fromabout 10 to about 15 weight percent.

The resulting solutions are then emulsified in an aqueous emulsifyingmedium in amounts such that the volume ratio of polymer solution toaqueous emulsifying medium is in the range of from about 1:0.5 to about1:1.5. By "aqueous emulsifying medium" is meant water containing anemulsifying amount of surface active agent, usually from about 0.2 toabout 5 weight percent based on the medium of the surface active agentpreferably from about 0.4 to about 1 weight percent. The concentrationof surface active agent is usually dependent on the volume ratio ofpolymer solution to aqueous medium, the viscosity of the polymersolution and the nature of the surface active agent. Surface activeagents which are particularly useful for successful practice of thisinvention are anionic emulsifiers or mixtures thereof with non-ionicemulsifiers. Examples of suitable anionic emulsifiers and non-ionicemulsifiers for this purpose are set forth in McCutcheon's Detergentsand Emulsifiers, Allured Pub. Co., Ridgewood, N.J. (1970 Annual).

Emulsification is effected by subjecting the combined polymer solutionand aqueous medium to high shear agitation conditions commonly employedin emulsifying polymer solutions in aqueous media. Emulsification issuccessfully accomplished using a colloid mill, a homogenizer or similarhigh shear dispersing device. The foregoing is merely illustrative, asthe particular method or means of emulsification is not critical anddoes not form an essential aspect of the present invention.

Following formation of the emulsion as described, the organic liquidsolvent and excess water are removed by conventional flashing techniquesto form a stable latex containing from about 25 to about 60 weightpercent of polymer solids wherein the particles of the dispersed phase(polymer particles) are colloidal size, e.g., in the range from about0.2 to about 5 micrometers, preferably from about 0.5 to about 2micrometers.

The desired crosslinking of the elastomeric copolymer is suitablyachieved by any conventional means including use of chemicalcrosslinking agents such as peroxygen compounds, e.g., t-butyl peroxide,and azo compounds, e.g., azobisisobutyronitrile; heat, irradiation orcombination thereof with irradiation being the preferred means.

Preferably irradiation is effected by subjecting a latex of theelastomeric copolymer to high energy radiation obtainable from any ofvarious high energy sources and can be of various types whether regardedas having corpuscular or wave form. By the term "high energy radiation"is meant a high intensity radiation having a voltage greater than 0.3mev., preferably from about 1 to about 10 mev. Representative types ofradiation suitable for the purposes of this invention are alpha rays,beta rays, gamma rays, X-rays, electron beams, high energy neutrons andthe like including radiations such as thermal neutron.

The dosage of radiation employed in the method of this invention is thatdosage which is sufficient to enable (strengthen) the elastomericcopolymer to remain as a colloidal size particulate during thesubsequent mechanical processing which is common in polymer fabrication,but which is less than that dosage which would destroy the elastomeric(rubbery) characteristic of the elastomeric copolymer. While suitabledosages vary with different elastomeric copolymers, it is generallyobserved that dosaages are suitably in the range from about 5 to about15 megarads, preferably from about 5 to 10 megarads. In order to avoiddestruction of the elastomeric character of the elastomeric copolymer,the dosage of radiation should be less than that providing a gel contentin the elastomeric copolymer of 65 weight percent, preferably less thanthat providing a gel content of 55 weight percent. The required highenergy radiation can be supplied from any of the well-known sources suchas cobalt or cesium sources. Examples are the electro-mechanical devicesfor producing high velocity particles such as a Van de Graaff generator,a resonant transformer, a cyclotron, a betatron, a synchrotron, asynchrocylotron, or a linear accelerator, X-ray tubes, and radioactiveisotopes emitting beta particles (high-velocity electrons) and/or gammarays. Irradiation of the colloidal-size particulate is carried out underconditions of temperature, etc., such that the particles retain theirdiscrete character and original colloidal size.

It is understood that, if a means other than irradiation is chosen toeffect crosslinking of the elastomeric copolymer, the degree ofcrosslinking should be that which is sufficient to enable theelastomeric copolymer to remain as a colloidal size particulate duringthe subsequent mechanical processing, but which is less than that whichwould destroy its elastomeric (rubbery) characteristic.

In one embodiment, the latex of the irradiated elastomeric copolymer canbe blended with a latex of the monovinylidene aromatic polymer. Thedesired polymer blend is recovered from the latex blend as describedhereinafter. Alternatively, the crosslinked block copolymer latex isrecovered from the latex in the form of the desired colloidal-sizeparticulate prior to blending with monovinylidene aromatic polymer.

Preferably, recovery is accomplished by conventional freeze-dryingtechniques which comprise the steps of (a) freezing the latex to afrozen mass, usually at temperatures from about -10° to about -20°C, and(b) drying the frozen mass under vacuum to remove water. Suitably, thelatex may be recovered by other conventional means such as by saltcoagulation, freeze-thaw destabilization, spray drying, etc. so long asthe recovered block copolymer remains in the form of discrete,colloidal-size particles or agglomerates thereof that can be readilyredispersed. When the latex is to be recovered by destabilizing, thecolloidal-size copolymer particles of the destabilized latex areseparated from the latex serum by filtration and then drying underconditions which allow the collected particles to remain as discreteparticles.

If not mixed in latex form, the non-elastomeric, monovinylidene aromaticpolymer and the colloidal size particles of the crosslinked elastomericcopolymer can be mixed or blended in any conventional manner whichprovides an intimate mixture of the components. Generally, blending inan internal mixer is preferred such as a Banbury, twin screw extruder,Brabender Plastograph, or the like, but an open mill can be employed.Also, mixing in an inert atmosphere can also be carried out as desired.Mixing temperatures can vary widely but will generally be in the rangeof from about 250 ° to about 600°, preferably from about 300° to about500°F, with mixing times in the range of from about 30 seconds to about30 minutes, preferably from about 1 to about 20 minutes.

The amount of elastomeric copolymer combined with the non-elastomericpolymer is at least that amount which measurably improves the impactstrength of the non-elastomeric polymer. Preferably, the amount ofelastomeric copolymer is in the range from about 5 to about 40 weightparts per 100 weight parts of the non-elastomeric polymer.

In order that the primary advantages of this invention over the priorart methods be realized, the blend or mixture of non-elastomeric polymerand the elastomeric copolymer is subjected to mechanical processing atsome point after the colloidal size particles of the elastomericcopolymer have been crosslinked. For the purposes of this invention, theterm "mechanical proocessing" means working the aforementioned blend toa degree sufficient to degrade (break down) colloidal size particles ofnon-crosslinked elastomeric copolymer. Examples of mechanical processinginclude the mechanical mixing or blending of the polymers in the drystate as described hereinbefore as well as fabricating the polymers intoarticles of desired shape by extrusion. It is found that furthercrosslinking of the elastomeric copolymer may occur during mechanicalprocessing. In most instances, this additional cross-linking isbeneficial. However conditions of mechanical processing should be chosenso that excessive crosslinking causing loss of elastomericcharacteristics does not occur.

The following examples are set forth to illustrate the invention andshould not be construed to limit its scope. In the examples, all partsand percentages are by weight unless otherwise indicated.

EXAMPLE 1

A polymer solution containing 10.2% of styrene/ butadiene (70/30) A-Bblock copolymer ([η]_(inh) = 1.4) in benzene is emulsified in watercontaining 3.0 percent sodium dodecyl benzene sulfonate and 0.83 percentnonylphenylethylene oxide (1 mole:9 moles) adduct based on copolymer byusing a homogenizer. The ratio of oil phase to aqueous phase is1.65:1.0. The resulting emulsion is then stripped of solvent andconcentrated to 53 percent latex polymer solid in a rotating evaporatorunder reduced pressure.

A portion (Sample No. 1) of the resulting latex is subjected to gammaradiation to a total dose of 10 megarads at a rate of 125 kilorads/hourusing a cobalt source. The crosslinked copolymer is recovered from thelatex as a colloidal-size particulate by freeze-drying technique using adry ice-methylene chloride bath to freeze the latex following whichwater is removed from latex under vacuum of 5 mm Hg.

Another portion (Sample No. A) of the latex which is not irradiated issimilarly recovered as a colloidal-size particulate.

The recovered portions are blended on compounding rolls for 6 minutes at160°C with polystyrene having a molecular weight of 280,000 at a ratioof 25 parts of block copolymer to 75 parts of polystyrene. The resultingpolymer blends are compression molded at 185°C for 5 minutes to givetest specimens which are measured for physical properties as indicatedin Table I.

Also for purposes of comparison (Sample No. B), 75 parts of thepolystyrene is blended with the non-irradiated block copolymer inmassive form and molded into test specimens which are tested forphysical properties as indicated in Table I.

                                      TABLE I                                     __________________________________________________________________________                                            Izod                                                                   Vicat  Impact                                    Yield Yield                                                                              Rupture                                                                             Rupture                                                                            Tensile                                                                              Heat   Strength,                             Sample                                                                            Strength,                                                                           Elong.,                                                                            Strength,                                                                           Elong.,                                                                            Mod.×10.sup.5,                                                                 Distortion,                                                                          ft.-lbs/                              No. psi (1)                                                                             % (2)                                                                              psi (3)                                                                             % (4)                                                                              psi (5)                                                                              °C (6)                                                                        in. notch (7)                         __________________________________________________________________________    1   4500  1.65 4435  1.71 3.31   102    1.89                                  A*  5521  1.83 5460  1.90 3.7    102    0.16                                  B*  4263  1.25 4185  1.26 3.59   101    0.28                                  __________________________________________________________________________     *Not an example of the invention.                                             (1)-(5) ASTM D-638.                                                           (6) ASTM D-1525-70.                                                           (7) ASTM D-256 Method A.                                                 

EXAMPLE 2

A polymer solution containing 10.2 percent styrene/butadiene (ABA type)block copolymer (˜15:70:15 ABA wherein A represents styrene block and Brepresents butadiene block) in benzene is emulsified in water containing3.0 percent sodium dodecylbenzene sulfonate and 0.83 percentnonylphenol-9-mole ethylene oxide adduct by using a homogenizer. Theratio of oil phase to aqueous phase is 1.54:1.0. The resulting emulsionis stripped of solvent and concentrated to 53.3 percent solids in arotating evaporator under reduced pressure.

A portion (Sample No. 1) of the resulting latex is irradiated andrecovered as described in Example 1. For purposes of comparison, anotherportion (Sample No. C) of the latex which is not irradiated is similarlyrecovered. The recovered portions are blended with polystyrene andmolded as described in Example 1 and tested for physical propertieswhich are recorded in Table II. A portion of the recovered irradiatedlatex is tested for gel content and gel swell ratio and the results arealso recorded in Table II.

                  TABLE II                                                        ______________________________________                                        Sample No.         1          C*                                              ______________________________________                                        Yield Strength,                                                               Yield, psi (1)     2537       No Yield                                        % Elongation (2)   1.26       None                                            Rupture Strength                                                              Yield, psi (3)     2512       2328                                            % Elongation (4)   13.8       1.6                                             Tensile Modulus × 10.sup.5, psi(5)                                                         2.96       2.04                                            Vicat Heat Distortion, °C(6)                                                              99         103                                             Impact Strength, ft. lbs/in(7)                                                                   1.86       0.82                                            Block Copolymer                                                               % Gel (8)          48         0                                               Gel Swell Ratio (9)                                                                              34         ∞                                         ______________________________________                                         *Not an example of the invention.                                             (1)-(7) Same as in Table I.                                                   (8)-(9) A sample of the irradiated latex of the block copolymer is dried      under vacuum at 60°C to constant weight and 0.5 g of dried polymer     is placed in 50 ml of toluene in a closed glass bottle for 70 hours at        room temperature. The amount of copolymer solubilized in the sample is        determined. The remainder is the gel content expressed as percent of the      original sample. The swell ratio is determined as the weight of the gel       fraction containing the imbibed toluene divided by the weight of the drie     gel fraction.                                                            

EXAMPLE 3

The aqueous polymer dispersion of the block copolymer as prepared inExample 1 [irridiated (Sample No. 1) and non-irradiated (Sample No. C)]is blended with a polystyrene latex (50 percent solids) in the ratio togive a polymer blend composition of 25 percent block copolymer/75percent polystyrene on dry weight basis. The polymer is isolated fromthese latex blends by removal of water under vacuum at 60°C. Theresulting polymer is compounded on rolls at 165°C for 7 minutes and thencompression molded into test samples at 185°C and 30 tons for 2 minutes.The resulting samples are then tested for physical properties which arerecorded in Table III.

                  TABLE III                                                       ______________________________________                                        Sample No.           1         C*                                             ______________________________________                                        Rupture Strength                                                              Yield, psi (3)       4362      4505                                           % Elongation(4)      1.7       1.6                                            Tensile Modulus(5), lbs/in.sup.2                                                                   3.2       3.45                                           Vicat Point (6), °C                                                                         104       103                                            Impact Strength (7), ft. lbs/in                                                                    1.45      0.41                                           Block Copolymer                                                               % Gel (8)            51        0                                              Gel Swell Ratio (9)  22        ∞                                        ______________________________________                                         *Not an example of the invention.                                             (3)-(7) Same as in Table I.                                                   (8)-(9) Same as in Table II.                                             

EXAMPLE 4

A 40 percent solids latex of styrene/butadiene (A/B type, 30/70) blockcopolymer ([η]_(inh) = 1.5) is made by emulsifying a 10 percent benzenesolution of the copolymer in accordance with the procedure of Example 1.The resulting latex is divided into three portions. The first and secondportions are subjected to γ-radiation in doses of 5 and 10 megaradsrespectively. Each of the three latex portions are blended withpolystyrene latex (50 percent solids) in the ratio to give a polymerblend composition of 25 parts of block copolymer per 75 parts ofpolystyrene on a dry weight basis.

The polymer component of each of the resulting blends is isolated byremoval of water under vacuum and dried. Each of the recovered driedpolymers is milled on compounding rolls for 7 minutes at 165°C and thencompression molded into test samples at 185°C and 30 tons pressure for 2minutes. The resulting samples are then tested for physical propertieswhich are recorded in Table IV.

                  TABLE IV                                                        ______________________________________                                        Sample No.     1         2*        C*                                         ______________________________________                                        Radiation Dosage, megarad                                                                    5         10        0                                          Tensile Strength                                                              Yield(1),lbs/in.sup.2                                                                        3303      0         2882                                       % Elongation(2)                                                                              1.67      0         1.43                                       Rupture Strength                                                              Break(3),lbs/in.sup.2                                                                        2269      3572      2509                                       % Elongation(4)                                                                              26.5      1.8       17.3                                       Tensile Mod.(5),lbs/in.sup.2                                                                 2.64×10.sup.5                                                                     2.71×10.sup.5                                                                     2.6×10.sup.5                         Vicat Point(6),°C.                                                                    104       104       104                                        Impact Strength (7),                                                                         3.79      0.48      0.91                                       ft.lbs/in                                                                     Block Copolymer                                                               % Gel (8)      10        78        0                                          Gel Swell Ratio (9)                                                                          60        17.8      ∞                                    ______________________________________                                         *Not an example of the invention.                                             (1)-(7) Same as in Table I.                                                   (8)-(9) Same as in Table II. Analysis of the block copolymer of Sample No     1 after milling on compounding rolls indicates a gel content of 41.2%.   

What is claimed is:
 1. A method for improving the impact resistance of a normally solid non-elastomeric monovinylidene aromatic polymer which is subjected to mechanical processing, said method comprising the steps of ( 1) subjecting colloidal size particles of an elastomeric block copolymer of from about 20 to about 80 weight percent of an elastomeric block of conjugated diene and from about 80 to about 20 weight percent of a non-elastomeric block of monovinylidene aromatic monomer to high energy irradiation sufficient to effect crosslinking of the block copolymer and (2) combining an amount of the resulting irradiated block copolymer particulate with the non-elastomeric monovinylidene aromatic polymer containing at least 70 weight percent of polymerized monovinylidene aromatic monomer wherein the amount of copolymer is sufficient to improve the impact strength of the non-elastomeric polymer, said crosslinking being sufficient to enable the block copolymer to remain in the form of colloidal size particles during mechanical processing, but less than that required to render the copolymer non-elastomeric.
 2. The method of claim 1 wherein the copolymer is subjected to high intensity radiation having a voltage greater than 0.3 mev employing conditions sufficient to crosslink the copolymer with a radiation dose in the range from about 5 to about 15 megarads.
 3. The method of claim 2 wherein the non-elastomeric polymer is polystyrene.
 4. The method of claim 3 wherein the elastomeric copolymer is a block copolymer of polystyrene block and polybutadiene block in which the block copolymer is present in the amount of from about 5 to about 40 weight parts per 100 weight parts of the non-elastomeric polymer.
 5. The method of claim 1 wherein the non-elastomeric polymer and elastomeric polymer are combined by blending together latexes of the polymers.
 6. A blend of non-elastomeric monovinylidene aromatic polymer and elastomeric block copolymer of conjugated diene and nonovinylidene aromatic monomer prepared by the method of claim
 1. 7. The method of claim 4 wherein the dose rate is 125 kilorads per hour using a cobalt source.
 8. The method of claim 4 wherein the copolymer is block copolymer having an A-B configuration with the A-block representing the polystyrene block and the B-block representing the polybutadiene block.
 9. The method of claim 8 wherein the dosage of radiation is less than that providing a gel content in the elastomeric copolymer of 55 weight percent.
 10. The method of claim 9 wherein the elastomeric copolymer contains from about 30 to about 70 weight percent of the polymerized diene and from about 70 to about 30 weight percent of the polymerized aromatic monomer. 